1. Field of the Invention
The present invention relates to transducers generally and, more particularly, to a novel element for vibrationally isolating the stacked piezoelectric ceramic plates found in some types of transducers and which is particularly useful in low-frequency, pressure gradient hydrophones.
2. Background Art
Piezoelectric elements are well known devices which change dimensionally when an electric potential is applied across them and which produce an electric potential when subjected to an external force.
A transducer of particular interest here is the stacked piezoelectric ceramic transducer having usefulness in hydrophones for detecting underwater sounds. Such a transducer includes a stack of piezoelectric ceramic plates that are electrically connected in parallel and provides an electric potential when an acoustic wave is received. Ideally, each ceramic plate is resiliently mounted so as to be able to vibrate completely independently of other plates. A particular problem exists with such hydrophones operating in a pressure gradient mode, that is, one in which the directional bearing of an acoustic wave, as well as amplitude thereof, is detected, in that they are highly susceptible to spurious resonances created by the interaction of the various components of the hydrophone itself.
It is desirable that such transducers be able to perform at low frequencies however, heretofore, conventional pressure gradient transducers have been limited, in that spurious resonances in the low frequency bands of interest are generated in the stack of transducer elements because of imperfect vibrational isolation. This is a particular problem when the band of interest approaches the resonant frequency of the components of the ceramic stack. Improved pressure gradient response free of spurious resonances would mean greater bearing accuracy. Freedom from spurious resonances is achieved when inter-element resonance is eliminated.
Designers of conventional hydrophone transducers first attempted to solve the problem of spurious resonances by interposing pads of resilient material between the surfaces of adjacent pairs of ceramic elements. This improved performance, but high bearing error existed on many hydrophones at a low frequency in the band where a spurious resonance resided. In a later development, there was disposed between each pair of ceramic elements a sandwich structure comprising a perforated metal annulus, with resilient annuli on either side thereof. While this construction alleviated the problem somewhat, the mounting still was too stiff and there still remained an undesirable vibrational signal level caused by the spurious resonances.
Accordingly, it is a principal object of the present invention to provide a piezoelectric ceramic stack transducer which can be operated at low frequencies without spurious resonances.
It is another object of the invention to provide an inter-element mounting between each pair of ceramic elements in such a transducer, which mounting decreases the stiffness of the coupling between the elements as compared to the previous resilient mounting, to increase vibrational isolation thereof.
It is an additional object of the invention to provide an improved ceramic stack for such transducers which can be used in existing installations.
It is a further object of the invention to provide such a ceramic stack which is easily and economically manufactured.
It is yet another object of the invention to provide an improved transducer stack, which stack particularly improves the response of pressure gradient hydrophones.
Other objects of the present invention, as well as particular features and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures.